Doped metal oxide semiconductor and thin-film transistor made therefrom and its application

ABSTRACT

The present application discloses a doped metal oxide semiconductor which is an indium tin oxide or indium tin zinc oxide semiconductor doped with a rare earth oxide. Even at a small doping amount, the oxygen vacancies could be suppressed as well as the mobility be maintained; critically, the thin-films made thereof can avoid the influence of light on I-V characteristics and stability, which results in great improvement of the stability under illumination of metal oxide semiconductor devices. The present application also discloses the thin-film transistors made thereof the doped metal oxide semiconductor and its application.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims priority from Patent Application No. 201910896373.4, filed in The People's Republic of China on Sep. 18, 2019, the contents of which are hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present application relates generally to a semiconductor fabrication specialized field, specifically relates to materials and device structures used to make the oxide semiconductor thin-film transistor applied in flat panel display devices and detectors, more specifically relates to doped metal oxide semiconductor and thin-film transistor made thereof and its application.

BACKGROUND

In the metal oxide semiconductor, the 5 s orbit of In³⁺ ions is the major electron transport orbit. Since the bond dissociation energy of the In³⁺ ions and O²⁺ ions is weak, abundant oxygen vacancies are existed in pure In₂O₃ thin-film, which is the main cause of the stability degradation of oxide semiconductor thin-film transistor. In general, it requires doping Zn²⁺ ions at a quantity comparable to In³+ in the oxide semiconductor thin-films to keep the amorphous structure.

Sn⁴⁺ ions provide excellent acid etching resist ability dues to the chemical inertia of Sn—O bonding. Abundant oxygen vacancies are existed in the traditional ITZO due to the weak bonds between In³⁺, Sn⁴⁺, and Zn²⁺ ions with O²⁻ ions, which is the main cause of the stability degradation of oxide semiconductor thin-film transistor. Furthermore, Sn⁴⁺ is usually used as a carrier source of electrons in the film to further increase the density of the carries in the thin-films. Therefore, it is an urgent issue to improve the electrical stability in the currently used ITZO material, especially the stability under illumination.

SUMMARY

In order to overcome the deficiencies of the prior art, the first object of this present application is to provide a doped metal oxide semiconductor with improved electrical stability, especially electrical stability under illumination.

The second object of this present application is to provide a thin-film transistor made therefrom.

The third object of this present application is to provide an application of the thin-film transistor.

To achieve the first objects, the present application provides the technical solutions as follows:

Doped metal oxide semiconductor, characterized in that the doped metal oxide semiconductor is a rare-earth oxide doped metal oxide; in which the metal oxide is indium tin oxide or indium tin zinc oxide; the rare-earth oxide is praseodymium oxide and/or ytterbium oxide; a doping molar ratio of praseodymium and/or ytterbium to the metal oxide is in a range from 0.002:1 to 0.4:1; there are recombination centers for photo-induced carriers generated in the doped metal oxide semiconductor.

That is, the doped metal oxide semiconductor provided by the present application is a rare-earth oxide doped semiconductor based on indium tin oxide-based metal oxide. Even at a small doping amount, the praseodymium oxide or ytterbium oxide could suppress oxygen vacancies as well as ensure the semiconductor with higher mobility. The stability of the indium tin oxide or indium tin zinc oxide semiconductor under illumination conditions was improved.

Preferably, a molar ratio of In⁴⁺ and Sn⁴⁺ in the indium tin oxide is in a range from 2:1 to 5:1. The indium tin oxide exhibits excellent mobility. After being doped by praseodymium or ytterbium at a relatively high ratio such as 0.4 molar ratio, it still maintains mobility value at 10 cm²/(V·s).

Preferably, a molar ratio of In⁴⁺, Sn⁴⁺, and Zn²⁺ in the indium tin zinc oxide complies with the following formulas: 0.2≤In/(In+Sn+Zn)≤0.8, 0.2≤Sn/(In+Sn+Zn)≤0.4, 0≤Zn/(In+Sn+Zn)≤0.5. For example, the indium tin zinc oxide with a molar ratio of In₂O₃:SnO₂:ZnO is 1.5:2:5 with a molar ratio of In⁴⁺, Sn⁴⁺, and Zn²⁺ is 3:2:5, or the indium tin zinc oxide with a molar ratio of In₂O₃:SnO₂:ZnO is 1:2:1, which with a molar ratio of In⁴⁺, Sn⁴⁺, and Zn²⁺ is 2:2:1. The indium tin zinc oxide within the molar ratio range could ensure a mobility value no less than 10 cm²/(V·s) after being doped by praseodymium or ytterbium at a molar ratio less than 0.1.

Preferably, a molar ratio of praseodymium or ytterbium doping to the indium tin oxide or indium tin zinc oxide is in a range from 0.02:1 to 0.40:1. The semiconductor with praseodymium or ytterbium doped at the said doping molar ratio range exhibits excellent suppression to photo-current effect and preferable mobility at the same time. More preferably, the doping molar ratio of praseodymium or ytterbium in rare-earth oxide to metal oxide could be in a range from 0.10:1 to 0.20:1. The metal oxide semiconductor with praseodymium or ytterbium doped at the said doping molar ratio exhibits relatively high mobility and good stability and its current switching ratio around 10⁷ to 10⁹.

To achieve the second objects, the present application provides the technical solutions as follows:

A thin-film transistor comprising an active layer. The active layer is made from the doped metal oxide semiconductor by a physical vapor deposition process, a chemical vapor deposition process, an atomic layer deposition process, a laser deposition process, or a solution process.

The present application discloses a thin-film transistor based on an active layer made by the doped metal oxide semiconductor. A variation of the turn-on voltage is (ΔV_(on)) less than 2 V between illumination condition (10000 nits) and dark condition. Preferably, ΔV_(on) is less than 1 V.

Preferably, the active layer is deposited by magnetron sputtering physical vapor deposition. The sputtering pressure is in a range from 0.1 to 0.6 Pa, an oxygen volume ratio in sputtering gas is in a range from 10 to 50 percent, and a substrate temperature is in a range from room temperature to 300° C. In specifics, the active layer is made by singer target sputtering or co-sputtering. An active layer with good uniformity, well adhesion and well quality could be deposited under the sputtering condition.

Preferably, the thin-film transistor further comprises a substrate, a gate electrode, a gate insulator layer, a source electrode and a drain electrode, and a passivation layer, the thin-film transistor is an etch-stop structure, a back-channel etch structure, or a top-gate self-alignment structure

Preferably, the passivation layer is a made of SiO₂ film or layered structure consisted of Si₃N₄ and SiO₂.

Preferably, the thin-film transistor further comprising a substrate, a gate electrode, a gate insulator layer, a source electrode, a drain electrode, and a passivation layer; the thin-film transistor is a back-channel etch structure which is etched by PAN-based wet etchant.

More preferably, the active layer is made by singer target magnetron sputtering.

Preferably, the substrate is rigid substrate such as alkaline glass, alkaline-free glass, quartz glass or silicon substrate, or some flexible substrates such as polyimide (PI), polyethylene naphthalene (PEN), polyethylene terephthalate (PET), polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethersulfone (PES) or foil and so on.

The gate electrode could be transparent conductive oxides some transparent conductive oxides such as ITO, AZO, GZO, IZO, ITZO, FTO and so on, or some metal such as Mo, Al, Cu, Ag, Ti, Au, Ta, Cr, Ni and so on or their alloy, or some metal and oxides layered material such as ITO/Ag/ITO, IZO/Ag/IZO and so on, or some layered metal such as Mo/Al/Mo, Ti/Al/Ti and so on.

The gate electrode could be grown by sputtering, electroplating, thermal evaporation and so on. The optimize method is sputtering, since the films exhibits excellent adhesion to the substrate, good uniformity and could be to realize large area.

The gate insulator layer could be one of SiO₂, Si₃N₄, Al₂O₃, Ta₂O₅, HfO₂, and Y₂O₃, and polymer organic film layer and so on, or layered structure consisting of those materials. The gate insulator layer could be a multi-layered structure, which not only exhibits excellent insulating performance but also improves interface characteristics between the active layer and gate insulator layer. In addition, the fabricating process of the gate insulator layer could be carried out by physical vapor deposition, chemical vapor deposition, atomic layer deposition, laser deposition, anodization or solution process and so on.

It should be particularly noted that in the fabricating process of the back-channel etch structural devices, a suitable etching selectivity ratio of the source and drain electrodes and the active layer should be carefully considered or otherwise, the device fabrication cannot be successfully achieved. H₂O₂-based solution is a preferable etching solution for the reason that the doped metal oxide semiconductor of the present application can effectively resist the wet etchant, which shows high etching selectivity ratio with the metal such as molybdenum, molybdenum alloy, molybdenum/copper, titanium/copper, etc. The metal oxide semiconductor material is unaffected by the etching solution, and the transistor made thereof exhibits excellent performance and good stability.

It should be particularly noted that the etching solution to etch the source electrode and drain electrode of the present application could be a stronger PAN-based wet etchant since that the doped metal-oxide semiconductor exhibits good corrosion resistance to strong acids.

To achieve the third objects, the present application provides the technical solutions as follows:

Application of the thin-film transistor in the display panel or detector.

The theories of this application are as following:

The doped metal oxide semiconductor in this application calls for only a small amount of rare-earth to suppress the oxygen vacancies effectively by doping praseodymium and/or ytterbium to indium tin oxide or indium tin zinc oxide, since the praseodymium or ytterbium exhibit high bond dissociation energy of 753.0(ΔH_(f)298, kJ/mol) and 715.0(ΔH_(f)298, kJ/mol), respectively. Therefore, the doping molar ratio could be cut down substantially to suppress the oxygen vacancies, which will improve the overlap of 5 s orbit of In³⁺ ion and reduce the effect to the mobility. Transistor made from the praseodymium and/or ytterbium doped indium oxide-based metal oxide semiconductor in this application could maintain high mobility of 30 cm²/(V·s).

Compared to the prior art, the beneficial effects of the present application are as follows:

This application discloses a praseodymium or ytterbium doped metal oxide semiconductor. The oxygen vacancies could be suppressed at a relatively low doping molar ratio. The doped metal oxide could be indium tin oxide or indium tin zinc oxide semiconductor. Comparing to traditional indium tin zinc oxide, the mobility of the transistor made thereof is almost unaffected.

After being doped by praseodymium oxide or ytterbium oxide, the doped metal oxide semiconductor will introduce impurity level near the Fermi level, therefor producing recombination centers for photo-induced carriers at 0.3 eV deviation range over the Fermi level. Those recombination centers could provide a rapid recombination channel that could avoid the photo-induced carriers to affect the I-V characteristics or stability, which could substantially improve the light stability of the metal oxide semiconductor.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic structural diagram of the thin-film transistor of example 9 and example 10.

FIG. 2 is a schematic structural diagram of the thin-film transistor of example 11.

FIG. 3 is a schematic structural diagram of the thin-film transistor of example 12.

FIG. 4 is the light-response curves of the thin-film transistors of example 9.

FIG. 5 is the light-response curves of the thin-film transistors of example 10.

FIG. 6 is the light-response curves of the thin-film transistors of example 11.

FIG. 7 is the light-response curves of the thin-film transistors of example 12.

FIG. 8 is PL spectrum and lifespan characterization of indium tin zinc oxide thin films with praseodymium doping at 0.1 molar or without praseodymium doping of example 7.

Where in: 01, substrate; 02, gate electrode; 03, gate insulator layer; 04, active layer; 05, etch-stop layer; 06-1, source electrode; 06-2, drain electrode; 07, passivation layer; 08, buffer layer.

DESCRIPTION OF EMBODIMENTS

Hereinafter, in conjunction with the accompanying drawings and specific embodiments, the present application will be further described. It should be noted that, in the case of non-collision, a new embodiment may be formed by any combination between the embodiments described below or between the technical features.

The following are specific embodiments of the present application, and the raw materials, equipment and the like used in the combined embodiments can be obtained by purchase unless otherwise specified.

Example 1: Ytterbium-Doped Indium Tin Oxide Semiconductor Materials

Example 1 discloses a group of metal oxide semiconductor materials. Those metal oxide semiconductor materials are ytterbium-doped indium tin oxides which are obtained by doping ytterbium oxide to the indium tin oxide as a light stabilizer.

Wherein, the molar ratio of In₂O₃:SnO₂ is 2:1 in indium tin oxide. The molar ratio of In, and Sn is 4:1. In the ytterbium-doped indium tin oxides, the doping molar ratio of ytterbium to indium tin oxide is 0.002, 0.020, 0.100, 0.200, 0.40 and 0.60, respectively.

Example 2: Ytterbium-Doped Indium Tin Zinc Oxide Semiconductor

Example 2 discloses a group of doped metal oxide semiconductor materials. Those doped metal oxide semiconductor materials are ytterbium-doped indium tin zinc oxides which are obtained by doping ytterbium oxide to the indium tin zinc oxide as a light stabilizer.

Wherein, the molar ratio of In₂O₃:SnO₂:ZnO is 1.5:2:5 in indium tin zinc oxide. The molar ratio of In, Sn, and Zn is 3:2:5. In the ytterbium-doped indium tin zinc oxides, the doping molar ratio of ytterbium to indium tin zinc oxide is 0.002, 0.020, 0.100, 0.200, 0.40 and 0.60, respectively.

Example 3: Praseodymium-Doped Indium Tin Zinc Oxide Semiconductor

Example 3 discloses a group of metal oxide semiconductor materials. Those metal oxide semiconductor materials are praseodymium-doped indium tin zinc oxides which are obtained by doping praseodymium oxide to the indium tin zinc oxide as a light stabilizer.

Where in, the molar ratio of In, Sn and Zn is 2:2:1 in indium tin zinc oxide. In the praseodymium-doped indium tin zinc oxides, the doping molar ratio of praseodymium to indium tin zinc oxide is 0.002, 0.020, 0.100, 0.200, 0.40 and 0.60, respectively.

Example 4: Praseodymium-Doped Indium Tin Zinc Oxide Semiconductor

Example 4 discloses a group of metal oxide semiconductor materials. Those metal oxide semiconductor materials are praseodymium-doped indium tin zinc oxides which are obtained by doping praseodymium oxide to the indium tin zinc oxide as a light stabilizer.

Wherein, the molar ratio of In, Sn and Zn is 4:3:3 in indium tin zinc oxide. In the praseodymium-doped indium tin zinc oxides, the doping molar ratio of praseodymium to indium tin zinc oxide is 0.002, 0.020, 0.100, 0.200, 0.40 and 0.60, respectively.

Example 5: Ytterbium-Doped Indium Tin Oxide Thin-Films

Example 5 discloses a group of metal oxide thin-films. Those ytterbium-doped indium tin oxide thin-films are deposited by magnetron sputtering from ytterbium doped indium tin oxide semiconductor materials in example 1.

Those metal oxide thin-films are made by co-sputtering of indium tin oxide target (ITO, doping molar ratio y=0) and ytterbium-doped indium tin oxide target (Yb:ITO, doping molar ratio y=0.60). The fabrication of thin-films with different doping molar ratio was obtained by changing the sputtering power.

Where in, the molar ratio of In₂O₃:SnO₂ is 2:1 in indium tin oxide target, which is marked as In(4)Sn(1). The molar ratio of ytterbium and indium tin oxide in ytterbium-doped indium tin oxide target is 0.60:1.00, which is marked as Yb:In(4)Sn(1). That is, a group of thin-films with gradient ytterbium doping molar ratios are obtained by changing the sputtering power. The ytterbium doping molar ratio value y to the indium tin oxide in this example is 0.002, 0.020, 0.100, 0.200, 0.40 and 0.60, respectively.

All the thickness of those thin-films is 30 nm. All those thin-films are made by sputtering in Ar and O₂ mixture gas in a volume ratio of 8:2 at 0.5 Pa. The chemical compositions of the thin-films are analyzed by X-ray photoelectron spectroscopy and transmission electron microscopy; portions with trace concentration are conjectured base on sputtering power.

Example 6: Ytterbium-Doped Indium Tin Zinc Oxide Thin-Films

Example 6 discloses a group of metal oxide thin-films. Those ytterbium doped indium tin zinc oxide thin-films are deposited by magnetron sputtering from ytterbium doped indium tin zinc oxide semiconductor materials in example 2.

Those metal oxide thin-films are made by co-sputtering of indium tin zinc oxide target (ITZO, doping molar ratio y=0) and ytterbium-doped indium tin zinc oxide target (Yb:ITZO, doping molar ratio y=0.60). The fabrications of thin-films with different doping molar ratios are obtained by changing the sputtering power. Where in, the molar ratio of In₂O₃:SnO₂:ZnO is 1.5:2:5 in indium tin zinc oxide target, which is marked as In(3)Sn(2)Zn(5). The doping molar ratio of ytterbium and indium tin zinc oxide in ytterbium doped indium tin zinc oxide target is 0.60:1.00, which is marked as Yb:In(3)Sn(2)Zn(5). That is, a group of thin-films with gradient ytterbium doping molar ratios are obtained by changing the sputtering power. The ytterbium doping molar ratio value y to the indium tin zinc oxide in this example is 0.002, 0.020, 0.100, 0.200, 0.40 and 0.60, respectively.

All the thickness of those thin-films is 40 nm. All those thin-films are made by sputtering in Ar and O₂ mixture gas in a volume ratio of 8:2 at 0.5 Pa. Those thin-films are annealed at 350° C. in air for 30 min before patterning. The chemical compositions of the thin-films are analyzed by X-ray photoelectron spectroscopy and transmission electron microscopy; portions with trace concentration are conjectured base on sputtering power.

Example 7: Praseodymium-Doped Indium Tin Zinc Oxide Thin-Films

Example 7 discloses a group of metal oxide thin-films. Those praseodymium-doped indium tin zinc oxide thin-films are deposited by magnetron sputtering from praseodymium doped indium tin zinc oxide semiconductor materials in example 3.

Those metal oxide thin-films are made by co-sputtering of indium tin zinc oxide target (ITZO, doping molar ratio y=0) and praseodymium-doped indium tin zinc oxide target (Pr:ITZO, doping molar ratio y=0.60). The fabrications of thin-films with different doping molar ratios are obtained by changing the sputtering power.

Where in, the molar ratio of In, Sn, Zn is 2:2:1 in indium tin zinc oxide target, which is marked as In(2)Sn(2)Zn(1). The molar ratio of praseodymium and indium tin zinc oxide in praseodymium-doped indium tin zinc oxide target is 0.60:1.00, which is marked as Pr:In(2)Sn(2)Zn(1). That is, a group of thin-films with gradient praseodymium doping molar ratios are obtained by changing the sputtering power. The praseodymium doping molar ratio value y to the indium tin zinc oxide in this example is 0.002, 0.020, 0.100, 0.200, 0.40 and 0.60, respectively.

All the thickness of those thin-films is 30 nm. All those thin-films are made by sputtering in Ar and O₂ mixture gas in a volume ratio of 8:2 at 0.5 Pa. The substrate temperature is room temperature.

The chemical compositions of the thin-films are analyzed by X-ray photoelectron spectroscopy and transmission electron microscopy; portions with trace concentration are conjectured base on sputtering power.

Example 8: Praseodymium Doped Indium Tin Zinc Oxide Thin-Films

Example 8 discloses a group of metal oxide thin-films. Those praseodymium doped indium tin zinc oxide thin-films are made by a solution process from praseodymium doped indium tin zinc oxide semiconductor materials in example 4.

This group of praseodymium doped indium tin zinc oxide thin-film are made as to the following steps:

First step: weighing indium nitrate, zinc nitrate, and tin chloride according to the molar ratio of In, Sn, Zn in 4:3:3 to obtain an indium tin zinc oxide matrix solution, which is marked as In(4)Sn(3)Zn(3). According to the specific doping molar ratio, a certain amount of praseodymium nitrate is added to obtain a group of praseodymium-doped indium tin zinc mixed solution with different doping molar ratio; In this example, the doping molar ratio value y of praseodymium to In(4)Sn(3)Zn(3) is to 0.002, 0.020, 0.100, 0.200, 0.40, and 0.60, respectively.

Second step: Ethanolamine, acetic acid, and ethylene glycol monomethyl ether solvent are added to each mixed solution respectively. Stirred at room temperature for 12 h and then standing for 12 h. The precursor solution was filtered, a light yellow transparent solution with a total concentration of indium, tin, zinc, and praseodymium is 0.06 mol/L, the concentration of ethanolamine is 1.2 mol/L, and the concentration of acetic acid is 1.3 mol/L is obtained.

Third step: The substrate that needs to be prepared an active layer is first treated with UV for 30 min to increase the surface adhesion. Then the substrate sheet is placed on a spin coater, and 60 μL of the precursor solution obtained in the first step is added to the substrate sheet by a pipetting gun, stand still for 20 s. Start the spin coater at a 500 rpm initial rotation speed for 3 s then a 3000 rpm later rotation speed for 40 s, so that the precursor solution is evenly coated on the substrate sheet; The substrate sheet was annealed at 120° C. for 10 min and then annealed at 500° C. for 120 min to obtain a praseodymium-doped indium tin zinc oxide film (named as Pr:ITZO) with a film thickness of 40 nm.

Example 9: Thin-Film Transistors

A group of thin-film transistors employing an etch-stop structure, whose schematic structural diagrams are shown in FIG. 1 , comprising: a substrate 01, a gate electrode 02 positioned on the substrate 01, a gate insulator layer 03 provided on the gate electrode 02 and the substrate 01, an active layer 04, an etch-stop layer 05, a source electrode and a drain electrode 06-1/06-2 that separately electrical connects to two sides of the active layer 04, and a passivation layer 07.

The active layer is made of thin-films in example 5.

The substrate 01 is made of glass and covered with silicon oxides buffer layer. The gate electrode 02 is a Mo/Al/Mo layered structure with a thickness of 50/200/50 nm to each layer, which is deposited by magnetron sputtering of corresponding material subsequently;

The gate insulator layer 03 is a Si₃N₄/SiO₂ layered structure with a thickness of 250/50 nm to each layer, which is fabricated by chemical vapor deposition (CVD). The Si₃N₄ layer electrical closes to the gate electrode 02 and the SiO₂ layer electrical faces to the active layer 04.

Both the etch-stop layer 05 and the passivation layer 07 are 300 nm SiO₂ layers that fabricated by CVD at 250° C.

Both the source electrode 06-1 and the drain electrode 06-2 in the source and drain electrode are Mo/Al/Mo layered structure with a thickness of 50/200/50 nm to each layer subsequently.

Example 10: Thin-Film Transistors

A group of thin-film transistors employing an etch-stop structure, whose schematic structural diagram are shown in FIG. 1 , comprising: a substrate 01, a gate electrode 02 positioned on the substrate 01, a gate insulator layer 03 provided on the gate electrode 02 and the substrate 01, an active layer 04, an etch-stop layer 05, a source electrode and a drain electrode 06-1/06-2 that separately electrical connects to two sides of the active layer 04, and a passivation layer 07.

The active layer 04 is made of thin-films in example 6.

The substrate 01 is made of glass and covered with silicon oxides buffer layer. The gate electrode 02 is a Mo/Al/Mo layered structure with a thickness of 50/200/50 nm to each layer, which is deposited by magnetron sputtering of corresponding material subsequently.

The gate insulator layer 03 is a Si₃N₄/SiO₂ layered structure with a thickness of 250/50 nm to each layer, which is fabricated by chemical vapor deposition (CVD). The Si₃N₄ layer electrical closes to the gate electrode 02 and the SiO₂ layer electrical faces to the active layer 04.

Both the etch-stop layer 05 and the passivation layer 07 are 300 nm SiO₂ layers fabricated by CVD at 230° C.

Both the source electrode 06-1 and the drain electrode 06-2 in the source and drain electrode are Mo/Al/Mo layered structure with a thickness of 50/200/50 nm to each layer subsequently.

Example 11: Thin-Film Transistors

A group of thin-film transistors employing a back-channel etch structure, whose schematic structural diagram are shown in FIG. 2 , comprising: a substrate 01, a gate electrode 02 positioned on the substrate 01, a gate insulator layer 03, an active layer 04 covers on an upper surface of the gate insulator layer 03 and corresponds to the gate electrode 02, a source electrode and a drain electrode 06-1/06-2 that separately electrical connects to two sides of the active layer 04, and a passivation layer 07.

The substrate 01 is made of rigid alkaline-free glass and covered with silicon oxides buffer layer.

The gate electrode 02 is a Mo/Al/Mo layered structure with a thickness of 50/200/20 nm to each layer.

The gate insulator layer 03 is a Si₃N₄/SiO₂ layered structure with a thickness of 250/50 nm to each layer. The Si₃N₄ layer electrical closes to the gate electrode 02 and the SiO₂ layer electrical faces to the active layer 04.

The active layer 04 is made of thin-films in example 7.

Both the source electrode 06-1 and the drain electrode 06-2 in the source and drain electrode are Mo/Al/Mo layered structure with a thickness of 50/200/20 nm to each layer subsequently. They are patterned under commercial available PAN-based wet etchant since the active layer exhibits excellent corrosion resistance to strong acid. The PAN-based wet etchant shows negligible damage to the active layer and excellent etching selectivity ratio as well as leaves almost no etching residue.

The passivation layer 07 is 300 nm SiO₂ layers fabricated by CVD at 250° C.

Example 12: Thin-Film Transistors

A group of thin-film transistors employing a top-gate self-alignment structure, whose schematic structural diagram are shown in FIG. 3 , comprising: a substrate 01, a buffer layer 08, an active layer 04, a gate insulator layer 03 and a gate electrode 02 positioned on the active layer 03, a passivation layer 07 covers on the active layer 04 and an upper surface of the gate electrode 02, a source electrode and a drain electrode 06-1/06-2 insert through the passivation layer 07 and connect to two sides of the active layer 04.

The substrate 01 is made of rigid glass.

The buffer layer 08 is made by plasma-enhanced chemical vapor deposition of SiO₂.

The active layer 04 is made of thin-films in example 8.

The gate insulator layer 03 is made of SiO₂ with a thickness of 300 nm. The gate electrode 02 is a Mo/Cu layered structure with a thickness of 200/20 nm to each layer which is deposited by magnetron sputtering.

The passivation layer 07 is a SiO₂/Si₃N₄ layered structure with a thickness of 200/100 nm to each layer.

Both the source electrode 06-1 and the drain electrode 06-2 in the source and drain electrode are Mo/Cu layered structure with a thickness of 200/20 nm to each layer subsequently.

In examples 9 to 12, the detailed descriptions of material or fabrication of each function layer are as follows:

There is no special restriction of the substrate in this present application, which means almost all the substrates well-known in this specialized field will do. For example, some rigid substrate such as alkaline glass, alkaline-free glass, quartz glass or silicon substrate, or some flexible substrates such as polyimide (PI), polyethylene naphthalene (PEN), polyethylene terephthalate (PET), polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethersulfone (PES) or foil and so on.

There is no special restriction of gate electrode material in this present application, which means almost all the gate electrode material well-known in this specialized field will do. For example, some transparent conductive oxides such as ITO, AZO, GZO, IZO, ITZO, FTO and so on, or grapheme, or some metal such as Mo, Al, Cu, Ag, Ti, Au, Ta, Cr, Ni and so on or their alloy, or some metal and oxides layered material such as ITO/Ag/ITO, IZO/Ag/IZO and so on, or some composite conductive film fabricated from layered metal such as Mo/Al/Mo, Ti/Al/Ti and so on.

The fabrication of the gate electrode could be carried out by deposition methods such as sputtering, electroplating, thermal evaporation and so on. The optimize method is sputtering since the film fabricated thereby exhibits excellent adhesion to the substrate, good uniformity and could be to realize large area.

Here, the employed structure of the gate electrode should be considered carefully depends on the desired technical parameters. For example, the transparent electrode is required at transparent display, which means the single-layer ITO or the ITO/Ag/ITO will do. In addition, high temperature resisted metal alloy should be applied when high-temperature processes are unavoidable at some special application.

There is no special restriction of the gate insulator layer material, which means almost all the well-known materials in this specialized field will do. Such as SiO₂, Si₃N₄, Al₂O₃, Ta₂O₅, HfO₂, and Y₂O₃, and polymer organic film layer and so on.

It should be noted that the composition of the gate insulating layer material may be inconsistent with the theoretical stoichiometric ratio. The gate insulator layer could be a multi-layered structure, which does not only exhibit excellent insulating performance but also improves interface characteristics between the active layer and gate insulator layer. In addition, the fabricating process of the gate insulator layer could be carried out by physical vapor deposition, chemical vapor deposition, atomic layer deposition, laser deposition, anodization or solution process and so on.

The thickness of the active layer in examples 9-12 could be in a range of 5 nm to 100 nm. Preferably, the carrier concentration of the metal oxide semiconductor film that makes the active layer should be small than 5×10¹⁹ cm⁻³.

The patterning process of the active layer is carried out by lithography that combining with wet or dry etching. The etching solution in the wet etching process including phosphoric acid, a mixture of nitric acid and glacial acetic acid, commercial oxalic-based etching solution, dilute hydrochloric acid etching solution, or hydrofluoric acid.

The patterning process of the source electrode and the drain electrode is carried out by lithography that combining with wet or dry etching. The etching solution in the wet etching process including phosphoric acid, a mixture of nitric acid and glacial acetic acid or a mixture based on aluminate-based peroxide. For dry etching, a plasma etching process may be selected, and the etching gas includes a chlorine-based or fluorine-based gas.

In the vacuum magnetron sputtering process that fabricating the active layer by metal oxide semiconductor material, single target sputtering or co-sputtering will do, preferably single target sputtering.

The active layer which is fabricated by single-target sputtering is more reproducible, more stable and its microstructure is easier to control; while the active layer which is fabricated by co-sputtering, the sputtered particles will be more affected by interference during the recombination process.

During vacuum sputtering deposition, the power source may be selected from radio frequency (RF) sputtering, direct current (DC) sputtering or alternating current (AC) sputtering, preferably alternating current sputtering in the industry.

During sputtering deposition, the sputtering pressure is optionally in a range of 0.1 Pa to 10 Pa, preferably in a range of 0.2 Pa to 0.7 Pa, more preferably 0.5 Pa, for it hard to maintain glow sputtering at a lower pressure, while the scattering of the sputtered particles during the deposition to the substrate will significantly increase, the energy loss increase, the kinetic energy reduce after reaching the substrate, and the defects of the thin-films made thereof will increase, which cause the performances of devices to be seriously affected at an excessive sputtering pressure.

During the sputtering deposition process, the oxygen partial pressure is in a range of 0 to 1 Pa, preferably in a range of 0.001 to 0.5 Pa, preferably in a range of 0.01 to 0.1 Pa. The oxygen volume ratio is in a range of 0.1 to 0.3, preferably in a range of 0.2 to 0.3.

In general, during the sputtering process that fabricating the active layer by the oxide semiconductor, the oxygen partial pressure shows a direct effect on the carrier concentration of the film, and oxygen vacancy-related defects may be introduced. Too low oxygen partial pressure may cause a serious mismatch of oxygen in the film and increase carrier concentration, while excessive oxygen vacancies will cause weak bonds and degrade the reliability of the devices.

The substrate temperature during sputtering deposition is preferably in a range of room temperature to 300° C., preferably in a range of 200 to 300° C.

During the deposition of the active layer, a certain substrate temperature can effectively improve the bonding mode of the sputtered particles after reaching the substrate, thus reducing the existence of weak bonds and improve the stability of the device. Of course, this effect can also be achieved by subsequent annealing processes and the like.

The thickness of the active layer is 2 to 100 nm, preferably 5 to 50 nm, and more preferably 20 to 40 nm.

There is no special restriction of the source electrode and drain electrode materials in this present application, which means that it can be selected arbitrarily from materials well-known in this specialized field affecting the realization of various desired structural devices. For example, some transparent conductive oxides such as ITO, AZO, GZO, IZO, ITZO, FTO and so on, or grapheme, or some metal such as Mo, Al, Cu, Ag, Ti, Au, Ta, Cr, Ni and so on or their alloy, or some metal and oxides layered material such as ITO/Ag/ITO, IZO/Ag/IZO and so on, or some composite conductive film fabricated from layered metal such as Mo/Al/Mo, Ti/Al/Ti and so on.

The fabricating process of the source electrode and drain electrode may be a sputtering method, thermal evaporation, or other deposition method. The sputtering deposition is the preferred method because the electrode fabricated thereby exhibits good adhesion to the substrate, good uniformity, and could be to realize large areas.

It should be particularly noted that in the fabricating process of the back-channel etch structural thin-film transistor, a suitable etching selectivity ratio of the source electrode and drain electrode and the active layer should be carefully considered or otherwise the device fabrication cannot be successfully achieved.

The etching solution in the wet etching process in the example of the present application is based on a conventional etching solution for metal in the industry, for example, an PAN-based wet etchant, for the reason that the metal oxide semiconductor material of the present application can effectively resist the wet etchant, which shows high etching selectivity ratio with the metal. The metal oxide semiconductor material is unaffected by the etching solution, and the transistor made thereof exhibits excellent performance and good stability.

Besides, the dry etching in the example of the present application is based on an industrial conventional etching gas, such as a chlorine-based gas or a fluorine-based gas, etc., which shows limited damage to the oxide semiconductor active layer of the present application, and the device fabricated thereof shows excellent performance and high stability.

There is no special restriction of the passivation layer materials in this present application, which means it can be arbitrarily selected from materials well known in the art such as silicon oxide, silicon nitride, aluminum oxide, hafnium oxide, tantalum oxide, hafnium oxide, and a polymer organic film layer and so on.

The composition of the passivation layer may be inconsistent with the theoretical stoichiometric ratio.

Besides, the insulator layer could be a multi-layered insulator layer, which not only exhibits excellent insulating performance but also improves interface characteristics between the active layer and the passivation layer. Also, the fabricating process of the passivation layer could be carried out by physical vapor deposition, chemical vapor deposition, atomic layer deposition, laser deposition, anodization or solution process and so on.

Here is a further description of the treatment process during the fabrication of the thin-film transistors in the examples of the present application.

Relatively speaking, the deposition rate of the thin-film which is fabricated by sputtering is faster because of the participation of high energy plasma. The thin-film does not have sufficient time to perform a relaxation process during deposition, which causes a certain proportion of dislocation and stress remained in the film. That requires a post-heat annealing treatment to further achieve the desired relative steady state and improve the properties of the film.

In the example of the present application, the annealing process is commonly employed after the deposition of the active layer or after the deposition of the passivation layer. For one thing, the annealing treatment after the deposition of the active layer can effectively improve the in-situ defects and improve the ability to resist damage in subsequent processes. For another thing, during subsequent deposition of the passivation layer, an “activation” process may be required due to the participation of the plasma and the modification of the reactive groups, to further eliminate the effects of interface states and some donor doping.

Besides, in the implementation of the present application, the method of interface treatment is not restricted to heat treatment, but also suitable to plasma processing interfaces, such as the interface between a gate insulating layer and semiconductor, or the interface between the active layer and passivation layer interface, etc. Through the interface treatment mentioned above, the performances of the device can be effectively improved, and the stability of the devices can be improved.

The thin-film transistors in examples 9 and 10 can be an enclosed structure consisting of the substrate, the gate electrode, the gate insulator layer, the active layer, the etch-stop layer, the source electrode, the drain electrode, and a passivation layer, or further comprising of a planarization layer, a reflective electrode, a pixel definition layer and so on, or even integrating with other devices.

The thin-film transistors in example 11 can be an enclosed structure consisting of the substrate, the gate electrode, the gate insulator layer, the active layer, the source electrode, the drain electrode and a passivation layer, or further comprising of a planarization layer, a reflective electrode, a pixel definite layer and so on, or even integrating with other devices.

The thin-film transistors in example 12 can be an enclosed structure consisting of the substrate, the active layer, the gate electrode, the gate insulator layer, the passivation layer, the source electrode, the drain electrode, or further comprising of a passivation layer, a pixel definition layer and so on, or even integrating with other devices.

Performance Testing and Characterizing

The Characterization of the Photo-Current Effect

The photo-current effects are characterized by irradiating the active layer of the thin-film transistors in examples 9 to 12 using commercially available LED lights with the intensities of 5000 nits. Variations of the turn-on voltage are calculated to evaluate the strength of the photo-current effects of the transistors. Wherein, the turn-on voltage is defined as the voltage of the gate electrode corresponding to the current between the source and drain electrodes at 10⁻⁹ A. The light-response characteristics are shown by the transistor's turn-on voltage variation amplitude (ΔV_(on)) between illumination condition and dark condition. The larger the value, the stronger the photo-current effects, and vice versa.

The Characterization of Negative-Bias Light Stability

The negative-bias light stability is characterized by irradiating the active layer of the thin-film transistors in examples 9 to 12 using commercially available LED lights with the intensities of 5000 nits with a voltage of −30 V was applied to the gate electrode of the transistor, and a 0 V bias was applied across the source and drain electrodes. The negative-bias light stability of the transistor under light irradiation can be obtained by testing the transfer characteristic curve of the device after some time interval. A large variation in thin-film transistors characteristics indicates poor stability, and vice versa.

PL Spectrum and Characterization of Lifespan

In principle, the active layer absorbs the energy under light irradiation to producing a photo-excitation process. Photo-excitation causes the electron to transition to higher excited state then releases the energy after the relaxation process to return to a lower energy level. The light radiation or luminescence in this process is called photoluminescence, which is called as PL. The un-doped and doped thin-film of the examples 5-8 are characterized using a commercial PL test system to understand the transport of photo-induced carriers in the thin-film after illumination. Besides, their recombination mechanism was further evaluated by testing their lifespan.

The specific parameters and performance of the thin film transistors groups 1-7 of example 9 are shown in the following table, and the photo-current effects and negative-bias light stability of the device when the doping molar ratio is 0.10 are as shown in FIG. 4(a) and FIG. 4(b), respectively.

TABLE 1 test 1 2 3 4 5 6 7 InSnO the molar ratio of In/Sn 4:1 ytterbium the value of y 0   0.002  0.020 0.10 0.20 0.40 0.60 active layer deposition method sputtering deposition O₂/(Ar + O₂)(%) 20   sputtering pressure(Pa) 0.5 substrate temperature (° C.) room temperature active layer annealing None treatment Composition substrate Glass buffer layer SiO₂ gate electrode Mo/Al/Mo gate insulator layer Si₃N₄/SiO₂ etch-stop layer SiO₂ source and drain electrode Mo/Al/Mo passivation layer SiO₂ After passivation annealing air-350° C. layer Performance threshold voltage V_(th)(V) −1.2  −1   −0.5  0.3  1.5  2.6  6.4  mobility μ(cm²/(V · s) 44.3  42.5  38.4  34.5  26.3  12.6  4.23 sub-threshold swing  0.12 0.12 0.16 0.18 0.25 0.28 0.46 SS(V/decade) current switching ratio 10⁹  10⁹   10⁹   10⁹   10⁸   10⁸   10⁶   I_(on)/I_(off) stability ^(a)) −− − + ++ ++ ++ ++ photo-current effect 14.60 6.30 1.70 0.12 0.05 0.01 0.01 −−^(Δ)Von(V) Note: ^(a)) −−poor; −, relatively poor; +, good; ++, excellent;

The specific parameters and performance of the thin film transistors groups 1-7 of example 10 are shown in the following table, and the photo-current effects and negative-bias light stability of the device when the doping molar ratio is 0.10 are as shown in FIG. 5(a) and FIG. 5(b), respectively.

TABLE 2 test 1 2 3 4 5 6 7 InSnZnO the molar ratio of 3:2:5 In/Sn/Zn Ytterbium the value of y 0    0.002  0.020 0.10 0.20 0.40 0.60 Active layer deposition method sputtering deposition O₂/(Ar + O₂)(%) 20   Sputtering pressure(Pa) 0.5 substrate temperature (° C.) room temperature Active layer annealing air-350° C. treatment Composition substrate Glass buffer layer SiO₂ gate electrode Mo/Al/Mo gate insulator layer Si₃N₄/SiO₂ etch-stop layer SiO₂ source and drain electrode Mo/Al/Mo passivation layer SiO₂ After passivation annealing air-350° C. layer Performance threshold voltage V_(th)(V) −0.2  0.1  0.5  1.1  1.7  3.6  7.2  mobility μ(cm²/(V · s) 15.8  15.1  13.3  11.5  7.4  5.8  0.4  sub-threshold swing 0.22 0.22 0.26 0.32 0.37 0.44 0.55 SS(V/decade) current switching ratio 10⁹   10⁹   10⁸   10⁸   10⁷   10⁶   10⁶   I_(on)/I_(off) stability ^(a)) −− − + ++ ++ ++ ++ photo-current effect 6.30 2.40 1.20 0.16 0.02 0.01 0.01 −−^(Δ)Von(V) Note: ^(a)) −−poor; −, relatively poor; +, good; ++, excellent;

The specific parameters and performance of the thin film transistors groups 1-7 of example 11 are shown in the following table, and the photo-current effects and negative-bias light stability of the device when the doping molar ratio is 0.100 are as shown in FIG. 6(a) and FIG. 6(b), respectively.

TABLE 3 test 1 2 3 4 5 6 7 InSnZnO the molar ratio of In/Sn/Zn 2:2:1 praseodymium the value of y 0    0.002  0.020 0.10 0.20 0.40 0.60 active layer deposition method Sputtering deposition O₂/(Ar + O₂)(%) 20   sputtering pressure(Pa) 0.5 substrate temperature (° C.) 200° C. active layer annealing None treatment Composition substrate Glass gate electrode Mo/Al/Mo gate insulator layer Si₃N₄/SiO₂ source and drain electrode Mo/Al/Mo passivation layer SiO₂ After passivation annealing air-350° C. layer Performance threshold voltage V_(th)(V) −1.3  −1.3  −0.5  0.4  1.2  2.8  4.6  mobility μ(cm²/(V · s) 38.9  37.2  32.5  29.3  21.7  14.6  5.3  sub-threshold swing  0.12 0.12 0.13 0.15 0.22 0.26 0.47 SS(V/decade) current switching ratio 10⁹  10⁹   10⁹   10⁹   10⁹   10⁸   10⁷   I_(on)/I_(off) stability ^(a)) − − + ++ ++ ++ ++ photo-current effect 10.60 4.80 1.30 0.05 0.02 0.01 0.01 −−^(Δ)Von(V) Note: ^(a)) −−poor; −, relatively poor; +, good; ++, excellent;

The specific parameters and performance of the thin film transistors groups 1-7 of example 12 are shown in the following table, and the photo-current effects and negative-bias light stability of the device when the doping molar ratio is 0.100 are as shown in FIG. 7(a) and FIG. 7(b), respectively.

TABLE 4 test 1 2 3 4 5 6 7 InSnZnO the molar ratio of 4:3:3 In/Sn/Zn praseodymium the value of y 0    0.002  0.020 0.10 0.20 0.40 0.60 active layer deposition method Solution process deposition substrate temperature 120° C. (° C.) active layer annealing 500° C. treatment Composition substrate Glass buffer layer SiO₂ gate insulator layer SiO₂ gate electrode Mo/Al/Mo passivation layer SiO₂/Si₃N₄ source and drain Mo/Al/Mo electrode After passivation Annealing air-350° C. layer Performance threshold voltage V_(th)(V) −0.6  −0.4  0.2  0.5  1.2  3.2  5.4  mobility μ(cm²/(V · s) 22.4  21.3  18.4  16.5  14.1  5.7  2.4  sub-threshold swing 0.12 0.12 0.16 0.19 0.21 0.26 0.43 SS(V/decade) current switching ratio 10⁹   10⁹   10⁹   10⁹   10⁹   10⁸   10⁷   I_(on)/I_(off) stability ^(a)) + + + ++ ++ ++ ++ photo-current effect 9.50 4.60 1.20 0.10 0.04 0.01 0.01 −−^(Δ)Von(V) Note: ^(a)) −−poor; −, relatively poor; +, good; ++, excellent;

As shown in Table 1-4, doping of rare-earth oxide shows obvious influences on the performances of the thin-film transistors. Firstly, the transistors without rare-earth doping exhibit the highest mobility, lower sub-threshold swing and negative threshold voltage but show strong photo-current effects, which means the performance of the thin-film transistors is degraded greatly under irradiation, such as negative threshold voltage shift and seriously degradation of sub-threshold swing. However, the photo-current effects of the transistors are suppressed by doping a certain amount of praseodymium or ytterbium oxides. Certainly, with the doping molar ratio of praseodymium or ytterbium increasing, the characteristics such as mobility will further degenerate, ΔV_(on) will further be decreased, and the photo-current effects will further to be improved. An excess praseodymium or ytterbium doping molar ratio could result in an obvious degeneration in mobility, which will largely restrict the application of those transistors even though they exhibit very weak photo-current effects. Since that, a suitable doping molar ratio should be considered carefully to balance mobility against photo-current effects. It can be seen from Tables 1-4 that the doping molar ratio of praseodymium or ytterbium doped to the metal oxide semiconductor, that is, the y value, is positively correlated with the threshold voltage, sub-threshold swing, and stability, while negatively correlated with the mobility, current switching ratio, and photo-induced current.

As shown in FIG. 4 to FIG. 7 , the transistors exhibit excellent light-response characteristics and negative-bias light stability after doping a certain amount of praseodymium oxide or ytterbium oxide to make the molar ratio reach 0.10:1, which greatly improved its potential in practical applications.

FIG. 8 is PL spectrum and lifespan characterization of indium tin zinc oxide thin-film with praseodymium doping at 0.1 molar or without praseodymium doping of example 7. As can be seen from FIG. 8(a), the un-doped thin-film shows a weak intensity of the emission spectrum while the overall strengths further decrease after praseodymium doping. As can be seen from FIG. 8(b), the lifespan test shows that the carrier lifespans are several nanoseconds which are meanly changed. It can be inferred that the relaxation process of carriers in the thin-film is mainly a recombination process of non-radiative transition, that is, a photo-induced carrier rapid recombination center exists in the film.

Considering the comprehensive performance, it can be seen from Table 1 that the active layer made from ytterbium-doped indium tin oxide, when the doping molar ratio y is in a range from 0.02 to 0.40, the overall performance of the transistor is better, and more preferably, when y is in a range from 0.02 to 0.20, the overall performance of the device is even better. When y is in a range from 0.10 to 0.20, the overall performance of the device is optimal.

As shown in Table 2, the active layer made from ytterbium-doped indium tin zinc oxide, when the doping molar ratio y is in a range from 0.02 to 0.40, the overall performance of the transistor is better, and more preferably, when y is in a range from 0.02 to 0.20, the overall performance of the device is even better. When y is in a range from 0.10 to 0.20, the overall performance of the device is optimal.

As shown in Table 3, the active layer made from praseodymium-doped indium tin zinc oxide, when the doping molar ratio y is in a range from 0.02 to 0.40, the overall performance of the transistor is better, and more preferably, when y is in a range from 0.02 to 0.20, the overall performance of the device is even better. When y is in a range from 0.10 to 0.20, the overall performance of the device is optimal. It should be noted that with the doping of yttrium oxide, the sub-threshold swing of the device exhibits a significant increase, which shows that the sub-threshold region of the device transfer curve tends “slowing down”, especially more obvious in the matrix of low carriers. It indicates that there is a new “trap” formed near the bottom of the conduction band of the active layer. The relevant test characterization of the bonded material indicates that its “quenching” effect on the photo-induced carriers is the main reason for the improvement of the photo-induced properties of the material.

As shown in Table 4, in the active layer formed by the praseodymium oxide doped indium tin zinc oxide semiconductor film, when y is in a range from 0.02 to 0.40, the overall performance of the transistor is better. When y is in a range from 0.02 to 0.20, the overall performance of the transistor is better. When y is in a range from 0.10 to 0.20, the overall performance of the transistor is optimal.

In overall, the doping molar ratio of the rare-earth oxide doped into the indium tin oxide or indium tin zinc oxide metal oxide semiconductor material is preferably in a range from 0.02 to 0.40 in the present application, more preferably in a range from 0.02 to 0.20, most preferably in a range from 0.10 to 0.20. In the most preferred doping molar ratio, the threshold voltage of the device is less than 2V, the sub-threshold swing is less than 0.40V/decade, the current switching ratio is about 10⁷ to 10⁹, the variation of the turn-on voltage is less than 0.2 V between illumination condition and dark condition, which means the photo-current effect is weak and the stability is excellent.

Those skilled in the art could make various other changes and the corresponding deformation according to the aspect and conception described above. All such changes and deformation should fall within the scope of the application as claimed in claims. 

What is claimed is:
 1. A doped metal oxide semiconductor, characterized in that the doped metal oxide semiconductor is a rare-earth oxide doped metal oxide; in which the metal oxide is indium tin oxide having an In:Sn molar ratio of 2:1 to 5:1 or indium tin zinc oxide having molar ratios of In, Sn, and Zn in ranges of: 0.2≤In/(In+Sn+Zn)≤0.8, 0.2≤Sn/(In+Sn+Zn)≤0.4, 0<Zn/(In+Sn+Zn)≤0.5; the rare-earth oxide comprises ytterbium oxide; an uniform doping molar ratios of ytterbium to the indium and tin or indium and tin and zinc of the metal oxide is in a range from 0.1:1 to 0.4:1; there are recombination centers for photo-induced carriers generated in the doped metal oxide semiconductor.
 2. The doped metal oxide semiconductor of claim 1, wherein the doping molar ratio of ytterbium to the indium and tin or indium and tin and zinc of the metal oxide is in a range from 0.20:1 to 0.40:1. 